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Chapter 10: The Indian Ocean
phytoplankton and contribute about 75% of both autotroph biomass and primary production (Gradinger et al., 1992). At the other end of the algal size spectrum, prominent
blooms of Oscillatoria erythraeum are frequent in the open parts of the Red Sea and are
perhaps the reason for its name; however, the significance of these blooms for basin-scale
carbon fixation and for the general pelagic ecosystem has not apparently been assessed.
The Red Sea is a net importer of zooplankton by the density-driven influx in the
south, though many species of expatriated Indian Ocean species do not long survive
the extreme conditions. It has been suggested that this nitrogen flux may partly balance
the negative nitrate balance associated with the two-layered pattern of exchange at the
entrance to the Red Sea. Obviously, zooplankton diversity is attenuated northward, and
in the extreme northern Red Sea relatively few oceanic species survive.
The effects on vertical profiles of plankton of the strong subsurface oxygen minimum
(O 2 < 13 ml liter
−1 ) in the Red Sea between 100 and 700 m have been investigated.
Whereas epiplankton avoid the layer so that biomass rapidly declines from 50 to 100 m
above the sharp pycnocline, the subsurface zooplankton biomass maximum lies within
the O 2 minimum layer. Here, diel migrants (e.g., Pleuromamma indica) have their daytime
residence depths, as this genus does elsewhere, along with apparently resting populations
of Rhincalanus nasutus (Weikert, 1984).
The extremely deep mixed layer, which may exceed 500 m in the Red Sea during the
winter mixing period, also has consequences for the vertical distribution of zooplankters
(Farstey et al., 2002). The epiplankton at this time is distributed homogenously throughout the layer, but not as a passive tracer: only the smaller organisms behave thus, while
the larger organisms (e.g., adult Pleuromamma sp.) retain their diel migration pattern,
although their nocturnal depth range is extended under such conditions.
Producer-consumer dynamics in the Red Sea are probably typical of extremely oligotrophic conditions. Protist consumption of small cells appears to exceed the consumption rate of mesozooplankton by about two orders of magnitude, taking heterotrophic
bacteria and small algae at turnover rates of 0.7–1.0 and 07–13 d
−1 , respectively. Curiously, removal of Synechococcus seems to suffer only very much lower levels of daily
mortality (Sommer et al., 2002). This observation may be related to the relatively weak
top-down control of larger nano-plankton by herbivores that characterizes the northern
Red Sea, compared with a strong bottom-up control of their population levels by nutrient
concentrations. Other observation in the southern Red Sea suggest removal by protists
of ∼50% of the standing stock and ∼100% of the primary production daily.
Satellite images suggest that the Arabian Gulf is relatively turbid throughout the
year with maximum clarity in the winter quarter (January–March) and we may expect
this to be due, in part, to suspended carbonate particulates. But it is not easy to be
confident, for there is a lack of studies of this region compared with the Red Sea and
much of the attention of researchers here has been on the consequences of coastal
development, urbanization, and oil discharges. Comparatively, the studies of regional
ecosystem dynamics are immature.
Nitrate concentration varies strongly seasonally and is undetectable at the end of
summer, whereas surface chlorophyll values in the range of 075–14 mg chl m
−3 occur
throughout the central part of the Gulf (Al-Saadi, 2001), with even higher local maxima
along the Trucial Coast (Dorgham and Moftah, 1989). Note that during the summer
complete mixing of the water column occurs over much of the Arabian Gulf, as noted
earlier. These chlorophyll values are almost an order of magnitude higher than those
in the adjacent Gulf of Oman at the same season. Once again, as in the Red Sea, it
is reported that in coastal regions the chlorophyll biomass accumulates to significantly
higher levels than in the central Gulf, and this observation is confirmed by the available
satellite imagery. The southern bight, into which the Qatari peninsula protrudes, sustains
very much higher concentrations of chlorophyll than elsewhere in many images.
Chapter 10: The Indian Ocean
phytoplankton and contribute about 75% of both autotroph biomass and primary production (Gradinger et al., 1992). At the other end of the algal size spectrum, prominent
blooms of Oscillatoria erythraeum are frequent in the open parts of the Red Sea and are
perhaps the reason for its name; however, the significance of these blooms for basin-scale
carbon fixation and for the general pelagic ecosystem has not apparently been assessed.
The Red Sea is a net importer of zooplankton by the density-driven influx in the
south, though many species of expatriated Indian Ocean species do not long survive
the extreme conditions. It has been suggested that this nitrogen flux may partly balance
the negative nitrate balance associated with the two-layered pattern of exchange at the
entrance to the Red Sea. Obviously, zooplankton diversity is attenuated northward, and
in the extreme northern Red Sea relatively few oceanic species survive.
The effects on vertical profiles of plankton of the strong subsurface oxygen minimum
(O 2 < 13 ml liter
−1 ) in the Red Sea between 100 and 700 m have been investigated.
Whereas epiplankton avoid the layer so that biomass rapidly declines from 50 to 100 m
above the sharp pycnocline, the subsurface zooplankton biomass maximum lies within
the O 2 minimum layer. Here, diel migrants (e.g., Pleuromamma indica) have their daytime
residence depths, as this genus does elsewhere, along with apparently resting populations
of Rhincalanus nasutus (Weikert, 1984).
The extremely deep mixed layer, which may exceed 500 m in the Red Sea during the
winter mixing period, also has consequences for the vertical distribution of zooplankters
(Farstey et al., 2002). The epiplankton at this time is distributed homogenously throughout the layer, but not as a passive tracer: only the smaller organisms behave thus, while
the larger organisms (e.g., adult Pleuromamma sp.) retain their diel migration pattern,
although their nocturnal depth range is extended under such conditions.
Producer-consumer dynamics in the Red Sea are probably typical of extremely oligotrophic conditions. Protist consumption of small cells appears to exceed the consumption rate of mesozooplankton by about two orders of magnitude, taking heterotrophic
bacteria and small algae at turnover rates of 0.7–1.0 and 07–13 d
−1 , respectively. Curiously, removal of Synechococcus seems to suffer only very much lower levels of daily
mortality (Sommer et al., 2002). This observation may be related to the relatively weak
top-down control of larger nano-plankton by herbivores that characterizes the northern
Red Sea, compared with a strong bottom-up control of their population levels by nutrient
concentrations. Other observation in the southern Red Sea suggest removal by protists
of ∼50% of the standing stock and ∼100% of the primary production daily.
Satellite images suggest that the Arabian Gulf is relatively turbid throughout the
year with maximum clarity in the winter quarter (January–March) and we may expect
this to be due, in part, to suspended carbonate particulates. But it is not easy to be
confident, for there is a lack of studies of this region compared with the Red Sea and
much of the attention of researchers here has been on the consequences of coastal
development, urbanization, and oil discharges. Comparatively, the studies of regional
ecosystem dynamics are immature.
Nitrate concentration varies strongly seasonally and is undetectable at the end of
summer, whereas surface chlorophyll values in the range of 075–14 mg chl m
−3 occur
throughout the central part of the Gulf (Al-Saadi, 2001), with even higher local maxima
along the Trucial Coast (Dorgham and Moftah, 1989). Note that during the summer
complete mixing of the water column occurs over much of the Arabian Gulf, as noted
earlier. These chlorophyll values are almost an order of magnitude higher than those
in the adjacent Gulf of Oman at the same season. Once again, as in the Red Sea, it
is reported that in coastal regions the chlorophyll biomass accumulates to significantly
higher levels than in the central Gulf, and this observation is confirmed by the available
satellite imagery. The southern bight, into which the Qatari peninsula protrudes, sustains
very much higher concentrations of chlorophyll than elsewhere in many images.